When drilling oil and gas wells, drilling fluids are used to maintain pressure at the bottom of the well to keep fluids from the formation from flowing into the well-bore. This drilling fluid may be water or oil based and is often referred to as mud or drilling mud. The drilling fluid is designed to have a density or weight so that the column of drilling fluid achieves a pressure slightly greater than the formation pressure encountered while drilling. As a well is drilled, the higher pressure from the well-bore forces fluid in the drilling fluid into the formation. Fluid from the drilling fluid driven into the formation by differential pressure causes the particles in the drilling fluid to be left on the wall of the well-bore. This layer of particles is the filter-cake. The build-up of this filter-cake on a well-bore causes difficulties. For instance, the filter-cake on the well-bore makes it difficult for cement to seal against the formation and can potentially compromise the zone isolation. Accordingly, special fluids, sometimes referred to as spacer fluids, are pumped into the well to remove the filter-cake. These spacer fluids may contain surfactants or emulsifiers to assist in this removal.
In addition to causing filter-cake to build, the fluid loss into the formation of the drilling fluids, as well as the fluid loss from the spacer, can also cause potential difficulties. For instance, these fluids lost into the formation can cause near well-born skin damage.
Because the drilling fluids and spacer fluids are variable, it is desirable to use drilling fluids that will minimize the build-up and spacer fluids that will maximize the removal of the resultant filter-cake. Coordinately, there is a need to use drilling fluids and spacer fluids that minimize the amount of fluid loss into the formation. The characteristics of the filter-cake are determined by the formation, the rheology of the drilling fluid, the temperature, the differential pressure at the surface of the formation (the difference between the pressure of the drilling fluid column and the formation), and by the flow velocity of the drilling fluid. Furthermore, the effectiveness of the spacer fluids is also affected by these same factors including the temperature, differential pressure, and by the flow velocity. The effectiveness of the spacer fluids is also affected by the factors that caused the build-up of the filter-cake. Accordingly, there is a need to simulate the build-up and removal of the filter-cake on the well-bore and the fluid loss into the formation so that more optimal drilling fluids and spacer fluids are used. And, since one of these characteristics is the flow velocity (i.e. the fluids are being circulated during operation), there is also a need to simulate the build-up and removal of filter-cake and fluid loss under dynamic conditions.
Therefore, there is a need in the art for a dynamic fluid loss cell apparatus and method, which is an apparatus and method that simulates the build-up and removal of filter-cake on the interior of a well-bore in oil and gas wells dynamically.